Tolerance induction to islets is a potential cure for insulin-dependent diabetes mellitus. T cell depletion plus a short course of 15- deoxyspergualin (DSG) induces stable tolerance to renal and islet allografts in non-human primates. Tolerance induction in this model is associated with dendritic cell maturation arrest and elevated plasma ILA and TGF-beta1. The immunoregulatory mechanisms mediating tolerance induction by this protocol are not known. Our long-range goal is to develop a uniformly effective protocol for tolerance induction that can be applied in clinical pancreatic islet transplantation for the treatment of insulin-dependent diabetes mellitus. The objective of this application, which is a logical step in the pursuit of that goal, is to determine the mechanisms mediating tolerance induced by peritransplant T cell depletion plus DSG in a murine model. The central hypothesis of the application is that dendritic cell maturation arrest and skewed TH2 cytokine profile, at the time of extensive T cell depletion, creates an environment that promotes functional inactivation of donor reactive T cells as they recover. This hypothesis has been formulated on the basis of strong preliminary data, which demonstrated the presence of immature dendritic cells during the two week period of DSG administration, and early elevation of plasma ILA and TGF-beta1. The rationale for the proposed research is that elucidating the underlying mechanisms of tolerance induction by this protocol will facilitate optimization, safe and rapid translation to clinical islet transplantation. The central hypothesis will be tested and the objective of the application accomplished by pursuing three specific aims: 1) Determine the role of dendritic cells in tolerance induction achieved by peritransplant T cell depletion plus DSG, 2) Identify cytokines mediating tolerance induced by peritransplant T cell depletion plus DSG, 3) Determine the mechanism(s) by which donor- reactive T cells are inactivated. The proposed work is innovative because it focuses on elucidating the immunoregulatory mechanisms of a novel, preclinical tolerance promoting protocol. The expectation is that the results will elucidate the early critical factors mediating tolerance induction and maintenance in this novel paradigm. The results will be significant because new ways to optimize the protocol for uniform effectiveness and safe translation to clinical transplantation are expected to be derived.